1. Field of the Invention
The present invention relates to a blowout preventer adapted to be included in a drill string wherein, upon the occurrence of an uncontrolled blowout of fluid (liquid and/or gas) in a subsea well, the blowout preventer is released in a manner to adopt an activated or set position stopping the blowout.
2. Description of the Related Art
Blowout preventers based on expandable packer elements are often used when drilling top hole sections, i.e. borehole sections near the seabed where high pressure shallow gas pockets may be encountered, to prevent uncontrolled blowout if the pressure downhole of the blowout preventer is higher that provided by the hydrostatic pressure caused by the drilling mud above the packer.
Packer elements of prior art blowout preventers of the above type, such as that disclosed in published PCT application WO 88/08917, are generally designed to be capable of completely isolating the borehole below the packer. In contrast to this, the present invention relates to a blowout preventer permitting a certain degree of controlled fluid leakage to the annulus above the packer, whereby drilling mud may be caused to fill up the entire borehole in an emergency situation associated with a dangerous blowout from the subsea well.
In order to detect any so-called "shallow gas" in top-hole sections in the upper seabed stratum, one or more "pilot holes" are often drilled. The pilot hole is drilled with a smaller diameter than fill bore diameter, in order to permit an increase of the downhole pressure by increasing the pumping rate of the drilling mud. Drilling pilot holes means added costs. Using a blowout preventer according to the invention eliminates the need of initially drilling a such pilot hole, while it also provides the necessary safety with regard to any high pressure zones that may be encountered. The blowout preventer according to the invention which is based on a packer means with a leak feature, is primarily intended for use in the above mentioned top-hole sections, where the borehole is not provided with casing.
When, during prior art drilling, the bit at the end of the drill string encounters a one of the formation having a higher pressure than that produced by the hydrostatic fluid column in the subsea well, any fluid (liquid and/or gas) present will be able to flow into the well and out of it, if no particular measure has been taken to prevent it.
Often, when drilling the upper section of a subsea well, no blowout preventer is used, and normally seawater is used as a drilling fluid when drilling the well. Therefore, when using this prior and conventional drilling technique, it is particularly important to ascertain that there are no high pressure zones to be encountered along the intended drilling path.
When drilling at greater sea depth, however, the formations in the upper strata of the subsea well are less consolidated than those at shallow waters.
The radial expansion capability of the type of packer that is used in prior blowout preventers is limited, and therefore such prior blowout preventers may not be capable of providing a satisfactory sealing around the wall of the wellbore annulus if the formation is unconsolidated or washed out to have a diameter larger than normal. Such insufficient sealing capacity of prior packer elements is uncontrollable and not to be compared with the intended and controllable leaking capability of the blowout preventer of the present invention, the latter permitting a fluid circulation that may be brought to cause the wellbore to be completely filled with drilling mud in an emergency situation such as a blowout.
Prior blowout preventers of the type having inflatable packer elements which are deflated when lowered into the well in their inactive ready position, require a higher internal pressure in their inflated, set position than the pressure prevailing below the packer element in the subsea well. This is due to the fact that the unset packer element itself resists being inflated.
As explained above such inflatable packing elements suffers from insufficient expandability upon inflation. On very deep waters formations might be encountered which are unconsolidated or weak, resulting in a non-uniform borehole wall having varying cross-section along a length thereof, which would necessitate an expansion of the inflatable packing element beyond that for which it is designed. Thus, prior packing elements are not capable of being sufficiently expanded.